Acute Myeloid Leukaemia Presenting as B12 Vitamin Deficiency When Multiple Horses Become a Zebra.

Vitamin B12 deficiency is a common finding in medical practice. It is easily treated with supplementation and typically has a favourable prognosis. In rare circumstances, it can hide a severe disease that should be promptly addressed. We report the case of an acute myeloid leukaemia presenting as an initially predictable B12 deficiency in a vegetarian patient with chronic gastritis. The supplementation rapidly corrected the deficit and the accompanying cytopenias. However, in the following month the cell counts fell once again, leading to the suspicion that other aetiology could be lying beneath the surface. Maintaining a normal peripheral blood smear, the bone marrow biopsy showed myeloblasts and extensive fibrosis compatible with the diagnosis of acute myeloid leukaemia. The neoplasm justified the vitamin deficit by excessive cellular turnover, a vicious cycle only uncovered after supplementation and that ultimately led to the patient's death. LEARNING POINTS: Vitamin B12 is a common aetiology of pancytopenia and is usually caused by gastric malabsorption.When supplementation does not correct the haematological deficit, central causes must be considered. Acute myeloid leukaemia is one possibility, but causes peripheral blood smear abnormalities in almost all patients. Diagnosis should include lumbar puncture and a thorough search for the aetiology; treatment is directed towards the aetiology.Neoplastic diseases should be always excluded when correction of the deficit does not resolve cytopenias.